Friction stir welding is a kind of precision processing technology． It has a very high demand for assembly accuracy. The performance of the weld can be directly affected by the assembly accuracy． Based on this background，the influences of butt clearance and thickness difference on the surface forming，the internal shape and the mechanical properties of the aluminum alloy were investigated． The results showed that FSW process had the capacity limit of 1． 1mm on the butt clearance． In the clearance of 0-1.1 mm，the mechanical properties of the joints were close to that of the defect free joints． When the clearance was higher than this limit，the performances of the joints would be significantly reduced due to the appearance of defects． FSW process also had a capacity limit to the thickness difference of the plate． When the plate thickness difference was less than the capacity limit，the mechanical properties of the joints would not change obviously． When the thickness difference of the plate was higher than the limit，welding quality would be decreased significantly.